The present invention relates to a heatable electric resistor for devices for measuring the flow speed or the mass flow of gases and liquids, which consists of an electrically insulating, plate-shaped substrate whose two largest surfaces are aligned parallel to the direction of flow of the gases or liquids and onto which on one of the two largest surfaces are applied contact surfaces and printed lines as thin metal layers separated from each other by separating cuts. As a result the printed lines run in a meandering fashion with their main direction perpendicular to the direction of flow and exhibit different widths. The electric heating power density in each printed line decreases in the same degree as the local heat flow density removed by the flow with increasing distance of the particular printed line from the edge of the substrate facing the flow, i.e. the leading edge.
In order to measure the flow speeds or the mass flow of gases and liquids, especially for measuring the intake air of internal combustion engines, anemometers are used in which two temperature-dependent electric resistors are connected together to at least two temperature-independent electric resistors in a bridge circuit. The one temperature-dependent electrical resistor is electrically heated and exposed to the flow of the fluid to be measured and the other measures the temperature of the fluid. An electric control circuit assures that the heated electric resistor is maintained at a constant difference temperature relative to the temperature of the fluid.
DE-PS 31 27 081 (U.S. Pat. No. 4,449,402) describes a heatable electric resistor for anemometers, consisting of an electrically insulating, plate-shaped substrate whose two largest surfaces are aligned parallel to the direction of flow of the gases or liquids and onto which on one of the two largest surfaces are applied contact surfaces and printed lines as thin metal layers separated from each other by separating cuts. The printed lines run in a meandering fashion with their main direction perpendicular to the direction of flow and exhibit different widths. The electric heating power density in each printed line decreases in the same degree as the local heat flow density removed by the flow with increasing distance of the particular printed line from the edge of the substrate facing the flow, i.e. the leading edge. By this way one achieves a short response time for changes of the flow velocity of the fluid to be measured.
The printed lines are preferably produced by placing separating cuts with a laser in the metal layer of a plate-shaped metal film resistor so that a meandering resistor pattern or path is created.
Only the battery is available in a motor vehicle for the current supply of the measuring bridge of an anemometer, which battery can exhibit a voltage which is distinctly lower than the nominal voltage, depending on its age, state of charge and condition. It must be assured even in this instance that the anemometer remains operational, that is, that it can receive a certain power.
For this reason, the total resistance of the bridge should be maintained as small as possible. However, certain resistance values can not be dropped below for design and electrical reasons. Today, bridge resistance of approximately 20 ohms are available and the attempt is being made to achieve resistance of 10 to 15 ohms.
The resistance of a measuring bridge is essentially determined by heatable resistor R.sub.H and temperature-independent resistor R.sub.L connected to it in series. The sensitivity of a constant-temperature anemometer is optimum when R.sub.H is approximately as great as R.sub.L. In practice, R.sub.L can be reduced to values of approximately 5 ohms whereas for reasons of geometry, R.sub.H was previously not able to be lowered under 10 ohms (resistance at 0.degree. C.).
A decrease of the electric resistance value of the heatable resistor is problematic because if the number of printed lines were reduced on the substrate, the homogeneity of the temperature distribution on the substrate would be worsened. In the case of contact surfaces in the region of plate edges which are opposite each other and run parallel to the direction of flow, only an even number of separating cuts perpendicular to the direction of flow and an odd number of printed lines are possible. In the case of a surface of approximately 9.times.2 mm.sup.2, approximately 11 ohms are obtained in a 1 .mu.m-layer of platinum with e.g. 5 printed lines and approximately 3.5 ohms with 3 printed lines. Thus, the electric resistance can only be varied in large steps and, moreover, only a very rough adaptation of the local production of heat to the heat flow density removed locally by the flow can be achieved, which results in a lengthened response time.